Molded plastic closures for pressure containing containers, such as soft drink bottles, have become prevalent and are in greater use in recent times.
For example, U.S. Pat. No. 4,016,996 to Aichinger et al, issued Apr. 12, 1977, discloses a plastic closure that is threaded on a narrow neck container. This closure has a cylindrical seal part that is intended to seal against the neck of the bottle and to prevent loss of gas from the container.
This and other plastic closures have experienced three common problems, the first of which is that they tend to "dome" upwardly due to the internal gas pressure exerting itself beneath the center of the cap. This doming of the cap has, in some instances, created cracks in the cap, resulting in leaking of gases or liquids from the container.
The "doming" may cause the seal to be pulled away or shifted on the container neck after initial sealing, resulting in the leaking of either gases or liquids from the container.
Most plastics, unless they are extremely thick, have a tendency to "creep" over a period of time under stress and, in the case of plastic closures for carbonated beverages, there is a fairly high degree of internal pressure being exerted on the underside of the cap. Thus, when "creep" may occur, there is a good possibility that the carbonation will not be maintained.
Another problem associated with the use of plastics in the bottling of carbonated beverages is the gas permeability of the plastics. While some plastics have very good gas barrier properties such as "Eval", an ethylene vinyl acetate resin produced by E. I. DuPont de Nemours, they are relatively expensive and therefore it is less desirable to use them in the large quantities that would be used in beverage bottle applications.
It is known that gas permeation through a plastic closure can contribute to as much as 6%-8% of the gas loss in the typical plastic closure-plastic bottle combination. In those cases where the plastic closures were used with a glass container, essentially all of the gas losses would be through the closure.